This invention relates to improvements in lift truck load-handling attachments having parallel load-handling arms (i.e. clamp arms, laterally-movable forks or other load-engaging implements), mounted on transverse slide members for opening and closing and thereby forcibly engaging the sides of loads. Such load-handling arms typically exert relatively high inward lateral force on a load by closing forcibly against the opposite exterior sides thereof to engage and lift it. In some cases the arms may also exert lateral force in an outward direction by opening forcibly against interior opposed surfaces of a load.
Prior load-handling devices of this type, referred to hereafter generally as load "clamps," are exemplified by U.S. Pat. Nos. 2,746,630, 2,782,065, 4,185,944 and 4,279,564, and by clamps manufactured by Kaup & Co. GmbH of Germany. All such load clamps have had certain characteristics in common to enable them to function properly. First, the slide members, and the slide guides upon which the slide members are movably mounted, have always been capable of withstanding large load moments, particularly moments about vertical axes in reaction to the lateral clamping force and, in some cases, moments about longitudinal axes where the load arms are forks and the lateral clamping force is concentrated adjacent the lower extremity of the slide assembly. These large load moments have required an extremely strong and rigid guide and slide assembly, most commonly provided by respective upper and lower pairs of guides and slides. Moreover, because the range of load widths is extremely large, a correspondingly large range of extensibility and retractability of the slide members has been required to enable the load arms to accommodate the different load widths. This in turn has required that the elongate slide members, the slide guides upon which they are mounted, and the piston and cylinder assemblies which move the slide members, all be of significant length, with the slide members longitudinally movable alternatively toward or away from one another in a mutually-overlapping longitudinal relationship. Furthermore, the forward protrusion of the slide members must be minimized so that the load can be carried by a counterbalanced lift truck at the smallest possible forward distance from the truck's front axle, in order to maximize the load-carrying capacity of the counter-balanced truck.
Several of the foregoing requirements of load clamps differ significantly from those of fork-positioning devices which also mount on lift trucks and move the forks alternately toward or away from each other laterally, as shown for example in Japanese patent publications 57-56694, 51-74355, and 51-129069, Russian patent publication 1011-496, and German patent 2929712. None of these fork-positioners is required to impose a high lateral clamping force on a load, nor to provide the range of extensibility and retractability required of a load clamp. Rather, such fork positioners normally move the forks laterally only when the forks are unloaded, so as to position them properly for engaging the bottom of a load, requiring little force and no resistance to the load moments mentioned previously. Moreover, the range of lateral fork positioning movements is relatively small compared to that of a load clamp, eliminating the need for long guides and piston and cylinder assemblies, as well as slide members which move toward or away from each other in a mutually-overlapping longitudinal relationship.
Both load clamps and fork positioners share a common need for enabling the lift truck operator to see the tips of the lower portions of the load-handling arms at varying elevations of the load-handling device, so that the operator can properly position the arms for engagement of a load. The primary visibility needs occur when the lower portions of the load-handling arms are at approximately 1450 millimeters above the ground (highway trailer height), at 1200 millimeters height (the normal interface between the upper and lower portions of a two-tiered load), and at ground level. However, because the foregoing two elevated positions require visibility through the vertical midsection of the load-handling device, it has been virtually impossible to effectively satisfy such visibility needs where load clamps are concerned because of the large slide, slide guide and piston and cylinder assemblies which must be provided to satisfy the high strength and range of motion requirements of load clamps. Space constraints have always required that the vertical midsections of such clamps be occupied by opposed piston and cylinder assemblies and/or other slide structure. Usually, a collection of separate hydraulic conduits (two per piston and cylinder assembly) also occupy the vertical midsection area. It has not been possible to create the extra space in a load clamp necessary to provide a vertical midsection free of obstruction without increasing the vertical height of the clamp to the point where the operator cannot see over the top of the clamp to the tips of the lower portions of the load arms when the clamp is lowered to the ground.
The foregoing visibility problem is not so severe where fork positioners are concerned, because the reduced loading and range of lateral motion of a fork positioner do not require that the slides and piston and cylinder assemblies be particularly large or vertically offset from each other in longitudinally-overlapping relationship, as is required with a load clamp. Accordingly it is much easier to create an unobstructed vertical midsection in a fork positioner than in a load clamp.
Another type of forklift truck load-handling device which has laterally openable and closable load-handling arms is a container handler such as that shown in European patent publication 055874. However such a device engages the load from its top rather than on its sides, so that the slides, guides and piston and cylinder assemblies are arranged in a horizontal plane instead of a vertical plane as in a load clamp. Accordingly such a device does not have visibility needs, or strength and protrusion requirements for the slides and guides, comparable to those of a load clamp.